The present disclosure relates generally to synchronous rectification, and more specifically to timing control of a synchronous rectifier used in a power supply.
Power suppliers are demanded for providing accurate output voltages, currents, or powers. Furthermore, to be eco-friendly to this world, power conversion efficiency, the ratio of output power to input power, has become one of the key issues that almost power supplies are required to improve. Several countries have further proposed or enforced regulations regarding power conversion efficiency requirements for certain applications.
FIG. 1 demonstrates a switching mode power supply 10 with a flyback topology. A pulse width modulator 14 turns ON and OFF a power switch 20 alternatively. When the power switch 20 is ON, the input voltage power VIN and input ground 26 together energize the primary winding of the transformer 18. When it is OFF, the energy stored in the transformer 18 is released via the secondary winding of the transformer 18 and rectified by a rectifying diode 12, to build output voltage power VOUT and output ground 28 and to power output capacitor 17 and load 16. With appropriate feedback control, the duty cycle of the power switch 20 could be modulated by pulse width modulator 14 so output voltage power VOUT meets its specifications required.
Transformer 18 provides secondary current ISEC from its secondary winding to power output capacitor 17 and load 16, and this secondary current ISEC has go through rectifying diode 12, which has a constant forward voltage (about 1V) and consumes power uselessly and inevitably. To save the power consumed by rectifying diode 12 and improve the power conversion efficiency, rectifying diode 12 has been replaced in some applications by a power switch with a very-low ON resistance, as demonstrated in FIG. 2, and this power switch is called synchronous rectifier 24 in the art. The timing to turn ON or OFF synchronous rectifier 24 must be well controlled to simulate the behavior of rectifying diode 12 in FIG. 1. For example, synchronous rectifier 18 should be turned OFF when power switch 20 is ON and the transformer 18 is being energized, and should be ON when the transformer 18 is releasing energy.
Synchronous rectifier 24 need be turned OFF timely. If synchronous rectifier 24 is turned OFF too early before transformer 18 completes discharging, the whole power supply system might not gain improvement in conversion efficiency. If synchronous rectifier 24 is turned OFF much later after the completion of discharging, output voltage power VOUT might reversely energize transformer 18. Much worse, if power switch 20 is switched to be ON when synchronous rectifier 24 has not been turned OFF, transformer 18 might explode, causing risk of fire.